Data communication systems need to accurately communicate as much data as possible within a constrained amount of frequency spectrum. This goal may be achieved by increasing the number of bits conveyed during any single symbol and/or by reducing the spectrum required to transmit successive symbols. The Nyquist class of pulse shapes minimize the spectrum required to transmit symbols, and these pulse shapes may be applied to higher-order QAM modulation schemes to convey a large number of bits per symbol. Nyquist pulses exhibit an amplitude of zero at a set of equally spaced time instants. By precisely positioning the zero points, intersymbol interference (ISI) and the severe performance degradation typically associated therewith may be avoided. While Nyquist pulses offer spectral efficiency, overall performance is highly sensitive to pulse shape. Any slight deviation from a precise Nyquist shape introduces catastrophic performance degradation. Due at least in part to this high sensitivity to pulse shape, the use of Nyquist pulses is conventionally limited to situations where a communication link's characteristics are precisely known and stable, such as in the telephony systems' land lines.
However, in mobile communications and other RF broadcast and free-space communication situations the communication links' characteristics vary dynamically and unpredictably. Nyquist pulse shaping schemes are typically far too frail for use in connection with free-space communication applications. Even slight multipathing can alter the Nyquist pulse shape enough to result in catastrophic performance degradation.
In addition, QAM modulation schemes employ amplitude modulation. When a QAM-modulated waveform passes through a power amplifier prior to transmission, this amplitude modulation is reproduced by the power amplifier. Desirably, the amplifier has a generally linear amplitude response so that the amplitude modulation is reproduced and so that a spectral regrowth phenomenon is avoided. The spectral regrowth phenomenon results from non-linear amplification and causes spectral energy to be regenerated outside the pass band of the amplifier's input signal.
In typical free-space communication systems, equipment expense and power consumption are important design parameters. Often times, these design parameters lead to the selection of transmitter high power amplifiers which consume a maximum amount of power at all input amplitudes and which exhibit a non-linear amplitude response near a saturation level. In other words, when operated at maximum power efficiency, the transmitter high power amplifiers have a non-linear amplitude response which is unsuitable for QAM. When operated in a linear range, the transmitter high power amplifiers exhibit serious inefficiencies in power consumption. Consequently, the linear amplification requirement of QAM forces the user to waste available transmitter power.
Desirable robustness and spectral efficiencies may be achieved through the use of modulation schemes that minimize amplitude modulation and force modulation signal phase changes to take place rather slowly. The minimization of amplitude modulation permits the use of high power amplifiers that exhibit non-linear amplitude responses. Consequently, desirable transmitter high power amplifiers may be used at maximum power efficiency.
A family of constant envelope (CE) frequency modulation schemes, including minimum shift keying (MSK), sinusoidal frequency shift keying (SFSK), Gaussian minimum shift keying (GMSK), and the like, minimize amplitude modulation and may force modulation signal phase changes to take place slowly. Of these frequency modulation schemes, GMSK appears to be the most spectrally efficient and otherwise desirable. These CE modulation schemes exhibit stable, robust performance characteristics which are well suited for the unpredictable and dynamic nature of free-space communication links.
Unfortunately, these CE modulation schemes suffer greatly from the detrimental consequences of intersymbol interference (ISI). These consequences include either a significant increase in bit error rate or a prohibitively complex receiver structure. Conventionally, the degradation in performance caused by the ISI associated with CE modulation schemes is so severe that the CE schemes are unable to accurately communicate data at higher orders, where multiple bits of data are transmitted during each symbol.
ISI is also experienced in QAM systems, although to a lesser extent. Attempts have been made to precode the data in QAM systems in an attempt to reduce the effects of ISI. However, these attempts require the modulation of amplitude. The amplitude modulation associated with amplitude precoding results in either an increased bandwidth or an ineffective solution when used with non-linear amplifiers.